PRIMITIVE VERTEBRATES. 7 



The Tunicata or Urochorda are remarkable forms, the 

 majority of which degenerate after larval life (Fig. 8). 

 In the larvae of all, and in a few adults which are neither 

 peculiarly specialised nor degenerate, we recognise some of 

 the fundamental characters of Vertebrates. Thus there is a 

 dorsal supporting axis (or notochord) in the tail region, a 

 dorsal nervous system, gill -clefts 

 opening from the pharynx to the 

 exterior, a simple ventral heart, and 

 so on. 



Of Balanoglossus and its allies 

 (Hemichorda or Enteropneusta) it is 

 still difficult to speak with confidence. 

 The possession of gill -clefts, the 

 dorsal position of an important part 

 of the nervous system, the occurrence 

 of a short supporting structure on 

 the anterior dorsal surface of the 

 pharynx, and other features, have led 

 many to place them at the base of 

 the Vertebrate series. 



Characteristics of Vertebrata. At 



this stage, having reached the base of the 

 Vertebrate series, we may seek to define a FIG. 8. Ascidian or 

 Vertebrate animal, and to contrast it with sea - squirt. After 

 Invertebrate forms. Haeckel. 



The distinction is a very old one, for 



even Aristotle distinguished mammals, birds, reptiles, amphibians, 

 and fishes as " blood-holding," from cuttle-fish, shell-bearing animals, 

 crustaceans, insects, etc., which he regarded as "bloodless." He was, 

 indeed, mistaken about the bloodlessness, but the distinctiveness of the 

 higher animals first mentioned has been recognised by all subsequent 

 naturalists, though it was first precisely expressed in 1797 by Lamarck. 



Yet it is no longer possible to draw a boundary line between Verte- 

 brates and Invertebrates with that firmness of hand which characterised 

 the early or, indeed, the pre-Darwinian classifications. We now 

 know (i) that Fishes and Cyclostomata do not form the base of the 

 Vertebrate series, for the lancelets and the Tunicates must also be in- 

 cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus, 

 and some other forms, have several Vertebrate-like characteristics ; 

 (3) that some of the Invertebrates, especially the Chaetopod worms, 

 show some hints of affinities with Vertebrates. The limits of the 

 Vertebrate alliance have been widened, and though the recognition of 

 their characteristics has become more definite, not less so, the apartness 

 of the sub-kingdom has disappeared. 



